The present invention relates to neutral centering mechanisms, and more specifically, to such mechanisms for use in returning the controls of a variable displacement hydraulic device from one of first and second operating positions, back to a neutral position.
Hydrostatic transmissions typically include variable displacement pump units, which include a fluid displacement mechanism, and some means operable to vary the displacement of the displacement mechanism in response to rotation of an input control member or control shaft.
Such variable displacement pump units are typically of the "over-center" type, i.e., the displacement mechanism may be displaced from the neutral position to either a first operating position (e.g., forward), or in the opposite direction, to a second operating position (e.g., reverse). In such devices, it has proven to be very difficult to design and manufacture commercially acceptable control devices, by means of which the control shaft of the pump may be returned from either the first or second operating position to "absolute neutral", i.e., a position of the control shaft corresponding to a zero displacement of the pump.
As is well known to those skilled in the art, one of the primary difficulties in designing and manufacturing such controls is that, typically, the ability of the neutral centering portion of the control to return the control shaft to absolute neutral is highly dependent upon the ability to maintain very close manufacturing tolerances on one or more of the parts of the neutral centering mechanism.
Another problem which has arisen with many such neutral centering mechanisms, for use in devices such as over-center pumps, is the need to make two or more separate adjustments within the centering mechanism, thereby compounding the tolerance sensitivity of the mechanism.